The invention relates to devices and methods for testing read/write memories with integrated redundancy in which test patterns are written to a memory array and subsequently read out and compared and, if possible, until no more defects are present, as long as word lines and column select lines are replaced by redundant lines.
On one hand, to date, self-test architectures integrated on a memory chip have only tested whether or not the associated chip is free from defects. On the other hand, a redundancy analysis in which the lines to be replaced are determined from defect addresses has been carried out to date with the aid of an external computer, since it has been the case heretofore that firstly all of the defective memory cells have been determined and only then has a corresponding replacement strategy been determined. Since a test method of that type requires inter alia a very large defect storage device, that method can only be expediently implemented with an external computer and, in practice, cannot be implemented as a built-in self-test.
It is accordingly an object of the invention to provide a read/write memory with an integrated self-test device and an associated test method, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and in which a complete self-test including redundancy analysis can be implemented without any significant external assistance.
With the foregoing and other objects in view there is provided, in accordance with the invention, a read/write memory, comprising at least one memory array having word lines and column select lines; and a self-test device monolithically integrated together with the at least one memory array, the self-test device having defect counters for the word lines and column select lines, redundancy consumption counters, stack storage devices for the word lines and column select lines to be repaired, a comparator connected to the stack storage devices for the word lines and column select lines to be repaired, a comparator connected to the defect counters for the word lines and column select lines, and a control unit.
In accordance with another feature of the invention, the control unit is programmed to perform the following functions in the following order:
a) initially setting all of the defect counters and redundancy consumption counters to zero;
b) comparing an information item written to the at least one memory array and an information item read from the at least one memory array and ascertaining a defective cell, with the comparators;
c) checking the word line and column select line associated with the defective cell to determine if it is already stored in the stack storage devices, and incrementing the counter associated with the respective word line and respective column select line only when one of the word line and the column select line has not yet been stored;
d) storing at least one of the word line and the column select line in the stack storage devices and incrementing the associated redundancy consumption counters if the associated defect counters exceed a repair threshold;
e) carrying out steps b) to d) for all prescribed test patterns and all cells of the at least one memory array or until one of the redundancy consumption counters overflows;
f) setting all of the defect counters to zero;
g) carrying out step b);
h) carrying out step c);
i) separately determining the word line and the column select line with the largest number of defects, with the comparators and the defect counters;
j) selecting one of the word line and the column select line with the largest number of defects;
k) storing the selected one of the lines with the largest number of defects in the stack storage devices and incrementing the associated redundancy consumption counter if the associated redundancy consumption counter has not yet overflowed;
l) carrying out step k) with the other of the lines selected according to step i) if the associated redundancy consumption counter has overflowed for the line selected according to step j);
m) terminating a test if the redundancy consumption counter overflows;
n) repeating steps f) to m) for all of the prescribed test patterns and for all of the memory cells of the at least one memory array; and
o) repeating step n) until all of the defect counters remain equal to zero or one of the redundancy consumption counters overflows.
In accordance with a further feature of the invention, there are provided fewer of the defect counters than the word lines and column select lines; an additional storage device for a maximum and an associated number of defects; and switches for assigning the defect counters to different groups of the word lines and column select lines; the control unit and the comparators determining and storing the word lines and column select lines with the largest number of defects within a group in the stack storage device instead of the word lines and column select lines with a previously largest number of defects, if the number of defects is greater than the previous largest number of defects, only the cells associated with the respective group of the word lines and column select lines being tested instead of all of the cells, and the test being repeated for all of the groups in the at least one memory array.
In accordance with an added feature of the invention, step j) is carried out by selecting one of the two lines having the largest number of defects.
In accordance with an additional feature of the invention, the defect counters have a different word width only for one word line and one column select line; the stack storage devices include a stack storage device for single-bit defects and a defect stack storage device; and the control unit is programmed to perform the following functions in the following order:
a) initially setting the defect counters and redundancy consumption counters to be equal to zero;
b) using the comparators for comparing information items written to and information items read out from the at least one memory array and ascertaining and storing a defective cell in the defect stack storage device;
c) checking the word line associated with the defective cell and the column select line with one of the comparators to determine if it is already stored in the stack storage device as a defective word line, as a defective column select line or as a single-bit defect, and skipping steps d) and e) if at least one of the word line and the column select line or the individual defect is already stored;
d) separately checking the word lines and column select lines associated with the defective memory cell and stored in the defect stack storage device for further defects and incrementing at least one of the defect counter associated with the word line and the defect counter associated with the column select line only when a respective further defect has not yet been stored in the stack storage device;
e) storing at least one of the word lines and the column select lines in the corresponding stack storage device and incrementing the associated redundancy consumption counter if the associated defect counter exceeds a repair threshold;
f) setting all of the defect counters to be equal to zero and repeating steps b) to e) for all test patterns and all memory cells of the at least one memory array; and
g) decreasing the repair threshold with one of the defect counters having a smaller word width and repeating step f) until, at a lowest repair threshold, the defect counters for all of the test patterns and all of the memory cells remain equal to zero or the redundancy counter overflows.
In accordance with yet another feature of the invention, in step d), further defects in the word line are sought only in the stack storage device for the column select lines to be repaired and further defects in the associated column select line are sought only in the stack storage device for the word lines.
In accordance with yet a further feature of the invention, there is provided a buffer storage device, the control unit being additionally programmed to initially perform the following functions prior to steps a) to g) in the following order:
A) storing the defective cells of one of the word lines in the buffer storage device with the word lines and column select lines;
B) checking to determine if a number of defects within one of the word lines exceeds a repair threshold prescribed by a size of the buffer storage device and accepting a buffer content in the defect stack storage device and clearing the buffer storage device if the repair threshold is not exceeded;
C) accepting a defective word line in the word line stack storage device, clearing the buffer storage device and incrementing the corresponding redundancy consumption counter if the repair threshold has been exceeded;
D) during testing of a next one of the word lines in steps A) to C), processing the defects in a previous one of the word lines in the defect stack storage device, incrementing the defect counter provided for the respective column select line and, in the event of a counter overflow, storing a relevant one of the column select lines in the stack storage device for the defective column select lines and incrementing the corresponding redundancy consumption counter; and
E) repeating steps A) to D) for all of the prescribed test patterns and all of the cells of the at least one memory array or terminating and already reporting all of the prescribed test patterns and all of the cells of the at least one memory array to be non-repairable, if the defect stack storage device or one of the redundancy consumption counters overflows.
With the objects of the invention in view, there is also provided a method for testing a read/write memory with a self-test device, which comprises storing at least one of word lines and column select lines to be repaired and excluding cells of the at least one of word lines and column select lines for further examinations; determining and examining at least one of the word lines and column select lines having the most defective memory cells not yet already detected by the stored at least one of word lines and column select lines before others of the at least one of word lines and column select lines; and ascertaining if a number of redundant lines still suffices and if no more defective cells are already present.
In accordance with another mode of the invention, there is provided a method which further comprises:
a) initially setting defect counters and redundancy consumption counters to zero;
b) comparing an information item written to a memory array and an information item read from the memory array and ascertaining a defective cell;
c) checking the word line and the column select line associated with the defective cell to determine if it is already stored and incrementing the defect counter associated with the respective word line or respective column select line only when the word line or the column select line has not yet been stored;
d) storing at least one of the word line and the column select line and incrementing the associated redundancy consumption counter if the associated defect counters exceed a repair threshold;
e) carrying out steps b) to d) for all prescribed test patterns and all of the cells of the memory array or until the redundancy has been consumed;
f) setting all of the defect counters to zero;
g) carrying out step b);
h) carrying out step c);
i) separately determining the word line and the column select line with the largest number of defects, with comparators and the defect counters;
j) selecting one of the word line and the column select line with the largest number of defects;
k) storing the selected line with the largest number of defects and incrementing the redundancy consumption counter if still-required redundancy is present;
l) carrying out step k) with the other of the lines selected according to step i) if no more redundancy is present for the line selected according to step j);
m) terminating a test if the corresponding redundancy has been consumed;
n) repeating steps f) to m) for all of the prescribed test patterns and for all of the memory cells of the memory array; and
o) repeating step n) until no more defects are present or the redundancy has been used up.
In accordance with a further mode of the invention, there is provided a method which further comprises:
a) initially setting all of a plurality of defect counters and all of a plurality of redundancy consumption counters to be equal to zero;
b) comparing information items written to and information items read out from a memory array and ascertaining and storing a defective cell;
c) checking the word line associated with the defective cell and the column select line to determine if it is already stored as a defective word line, a defective column select line or a single-bit defect, and skipping steps d) and e) if it is already stored;
d) separately checking buffer-stored word lines and column select lines associated with the defective memory cell for defects and incrementing at least one of the defect counter associated with the word line and the defect counter associated with the column select line only when the respective defect has not yet been stored;
e) storing at least one of word lines and column select lines and incrementing the corresponding redundancy consumption counter if the associated defect counter exceeds a repair threshold;
f) initially setting all of the defect counters to be equal to zero and repeating steps b) to e) for all test patterns and all memory cells of the memory array; and
g) decreasing the repair threshold and repeating step f) until, at a lowest repair threshold, no more defects occur or one of the redundancy consumption counters overflows for all of the test patterns and all of the memory cells.
In accordance with a concomitant mode of the invention, there is provided a method which further comprises initially performing the following steps in the following order, prior to steps a) to g):
A) storing the defective cells of one of the word lines with a word line and a column select line in a buffer storage device;
B) checking to determine if a number of defects within a word line exceeds a repair threshold and, if the repair threshold is not exceeded, accepting a content of the buffer storage device in a defect stack storage device and clearing the buffer storage device;
C) storing a defective word line, clearing the buffer storage device and incrementing the corresponding redundancy consumption counter if the repair threshold has been exceeded;
D) testing the next word line in steps A) to C) while processing defects of a previous word line in the defect stack storage device, incrementing the defect counter provided for the respective column select line, storing the relevant column select line and incrementing the corresponding redundancy consumption counter, in the event of a repair threshold being exceeded; and
E) repeating steps A) to D) for all of the prescribed test patterns and all of the cells of the memory array or terminating and already reporting all of the prescribed test patterns and all of the cells of the memory array to be non-repairable if the defect stack storage device or one of the redundancy consumption counters overflows.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a read/write memory with a self-test device and an associated test method, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.